Conventional films of plastics materials are generally essentially non-porous. Thus they are commonly used to provide impermeable barriers. A thin film can be of good mechanical strength.
Porous webs of fibrous materials are well-known, e.g. papers and both woven and non-woven webs. Such materials have pores with wide size ranges. Some pores will be very large, so that barrier properties will be significant only if the materials are thick, so that the pores are long and convoluted. Thin webs will have poor barrier properties and are also likely to have poor mechanical strength (particularly with non-woven webs).
In the production of non-woven webs, an initially-produced loose web is commonly compacted by calendering. This may be carried out under conditions of temperature and pressure such that there is some bonding of fibres, thus producing a stabilised fibrous web, e.g. as described in EP-A-0116845. That document also discloses the treatment of polyethylene fibre webs under more severe conditions to convert them to impermeable films. This is not generally a useful technique. If a film of a plastics material is required, it is cheap and easy to produce it directly from a melt (by extrusion and, if necessary, stretching). It is more expensive to extrude fibres and convert these to a non-woven web, and a further conversion step would add to the expense.
Composite webs with a film bonded to a fibrous web are also known. They may be treated so that the film becomes apertured, giving the composite some limited permeability (e.g. U.S. Pat. No. 4,684,568: treatment by calendering; U.S. Pat. No. 4,898,761: needling of the film). Such materials have low permeability, are quite expensive to produce, and are of limited applicability.
We have appreciated that there would be many potential uses for porous films with controlled pore sizes, particularly if they were `breathable`, i.e. of substantial permeability. For example, medical and surgical items are generally supplied in a sterile state enclosed within individual packages fabricated in part from porous materials. Such porous materials are of necessity permeable to gases and vapours so as to permit sterilisation of the item (after packaging) by means of steam or a gas such as ethylene oxide. Furthermore, permeability to air is important to allow the application of a vacuum during sterilisation, to facilitate the packaging process and to limit the air volume around the packaged item. However, in spite of this air permeability the material must act as an effective barrier to the passage of micro-organisms to that the packaged item remains sterile. Conventional polymeric films possess little or no gas permeance and consequently their use is limited to forming a non-porous part of a medical package.
Other examples of potential uses for porous films with controlled pore sizes include filtration (e.g. of particles from liquids) and controlled release of vapours.
The natural assumption is that if you subject a web having pores of a range of sizes to sufficient heat and pressure, the result will be a general reduction in the sizes of the pores, leading to a progressive closing of pores, starting with the smallest ones. It could not have been predicted whether it would be possible to isolate part-treated materials (before complete closure of all pores). But there was no incentive to try, since the materials would not be expected to be of interest. They would be expected to have lost most of their permeability, owing to the general reduction in pore sizes; they would also be expected to have lost most of their barrier properties, since the inevitable reduction in thickness would have removed most of their ability to act as depth filters, whereas the pore size distribution would mean that they still possessed a proportion of relatively large pores.